Most semiconductor devices are built up using a number of material layers. Each layer is patterned to add or remove selected portions to form circuit features that will eventually make up a complete integrated circuit. The patterning process, known as photolithography, defines the dimensions of the circuit features.
The goal of the patterning process is to create circuit features in the exact dimensions required by the circuit design and to place them in the proper locations on the surface of a semiconductor wafer. Perfect alignment is an ideal that cannot be achieved in practice. Instead, the various layers of an integrated circuit will be misaligned to some extent. Such misalignment is termed "mask misalignment" because misaligned mask images are the source of the error. When circuits fail during fabrication, it is desirable to determine whether the source of the failure is incorrect mask alignment.
There are a number of conventional methods of detecting mask misalignment. For example, U.S. Pat. No. 5,770,995 to Masayuki Kamiya describes a structure that identifies misalignment between a conductive layer and a contact window layer. The disclosed structure indicates the direction of mask misalignment but does not provide an accurate measure of the extent of misalignment. Each of U.S. Pat. No. 4,386,459 to David Boulin and U.S. Pat. No. 4,571,538 to Pei-Ming Chow describe structures that indicate both the direction and extent of mask misalignment. However, the disclosed structures rely upon process-sensitive circuit parameters to produce accurate misalignment data. For example, misalignment data provided by both the Boulin and Chow structures is sensitive to line-width and resistivity variations. There is therefore a need for a mask-alignment detection structure that accurately indicates the direction and extent of mask misalignment, despite process variations.
The above-mentioned U.S. Patents provide useful background information, and are therefore incorporated herein by reference.